My 2005 Sanger with Black Scorpion has been running poorly all season. I had a mechanic out to the lake. He ran his computer. Nothing was wrong according to his computer, but he agreed that I had a misfire. The misfire happens under load at 1500-3000 rpms. The boat has new cap, rotor, and plugs. Prior to coming to the lake, the mechanic did a drop down test to check the valves, and checked intake manifold pressure at his shop. No problems with either. He has been working with Mercrusier and his shop is certified. They came to the conclusion that we need to have the ECM recalibrated. I asked if I should run it through the season and send the ECM in this winter. They said this would be a good plan. I have had other mechanics look at the boat and have found no problems. The backfire continues... Any advice?

Don't know the timing numbers, but from what the mechanic said, everything should be within spec. I have not measured resistance in the plug wires. I was considering replacing all the wires. The plugs that came out looked like it had been running a little lean (according to the mechanic), but nothing bad.

I will check the distributor/cap/rotor etc for anything I can see. I've had it off and everythhing looks clean. I did notice that the distributor cap is 90 degrees off from where I'd expect it to be. It fires 1-8-4-3-6-5-7-2. I thought 1 would go just to the right of 12:00. The way the cap is on, 1 is just above 9:00. The mechanic says this won't cause any problems.

Thanks for the input. I'll go back through the wires/distributor/etc..

Where it starts the firing order at is not a big deal as long as the order is correct.

If its back firing under load like that, and you said the plugs looked like it was running lean, That is what happens when the air-fuel mixture has insufficient fuel and whenever the timing is too advanced, then it is probably in your timing or a malfunctioning distributor.

Reprogramming the ECM probably won't do squat but cost you some money, it would be the last resort thing to do. Check your distributor over real good and check your timing.

The mechanics computer won't tell him anything if it's not throwing a code of some kind.

Robby said the distributor base was plastic, then get a new distributor, The manufacture should know better than that. I would be all up in indmar if that was my engine, I'd try my best to get them to replace the distributor.

I checked the resistance on the plug wires today. All were reading 13-18 with my ohmeter set at 200K. I assume this is such a small amount of resistance that it is normal.

The motor I have is a black scorpion Mercruiser. I checked the distributor and the base plate is metal. There was very little corrosion as the cap is new. Not sure where to go from here. There is a misfire that is intermittant. It happens when the boat is decelerating. It runs like a top when accelerating. New plugs, new rotor, new cap, I assume good wires. What's next. Is it possible the ECM does need recalibration? Mercruiser will do it for free, but it takes 4-6 weeks. Sounds like something to send in during the winter. Comments?

It might periodically misfire under load, but not often. I have perfect pass, and as the throttle is reducing, it has a misfire. It isn't continuous. Usually just when the throttle starts being reduced. The mechanic checked fuel pressure. It was good. Had the injectors cleaned. I put on a new fuel filter. There's no water in the fuel water seperator. Mostly new ignition parts mentioned above. Any ideas?

I should mention that the boat comes out of the hole just fine. We fill the ballast and have 5-6 people in the boat and it gets up on plane normally. Misfire usually happens as the throttle is backing off.

If it is just on decel, then start looking for intake and exhaust leaks, make sure everything it tight and sealed up real good and check the timing.

I would call and talk to mercruiser about it and see if there is a history of the ECM needing a recal for this issue. It just seems odd, cause there code just doesn't get corrupted unless you or somebody else has messed with it, and it's not throwing any codes.

Running a engine that is back firing it not good, even the smallest of back fires can hurt your engine. A back fire while on decel happens when both valves are open at the same time(called valve overlap)and the unburnt fuel is sucked back into the cylinder thus you get a pop(back fire).

Get some carb cleaner and start the engine, spray carb cleaner around the intake and the injectors and see if the idle goes up, if it does then you have a leak. Check the exhaust for leaks also, if there is no back pressure then it can cause a misfire also. Have you modified anything on this engine?

Just giving you some little cheap things to try before you have to look into it more. I always start with the simple stuff first.

The tune up certainly didn't hurt it, but almost every mechanic will tell you the same thing when ask questions. Tune it up and that should fix it, they say that cause they really have no clue unless you bring it to them what is wrong, and 50% of the time, a tune up will fix it.

Dorian, a leak down was already done as stated earlier. No problems found. This would clear any valve or gasket sealing issues. Also if it was a valve or gasket issue it would not be intermittant. I still want to know about the timing. I believe the answer is most likely not in fuel but rather ignition. I also would like to clarify. Is it a misfire or backfire? A backfire will be heard as a pop out of the air cleaner/spark arrestor or the exhaust. A misfire will be the engine running on 7 cylinders, or less, momentarily.

I used carb cleaner to check for intake leaks. I also had the intake pressure checked by a mechanic. No leaks.

They did what they call a manual drop down test to check valve leaks. I think the idea was to pull a spark plug wire and check for RPM drop at a certain RPM. Any wire that doesn't drop as much could have a valve leak. I don't remember starting RPM, but pulling plug wires resulted in 1530-1570 RPMs for all cylinders. From this Mercruiser said I probably don't have a valve leak. Does this sound reasonable to anyone that knows about drop down tests?

Nothing has been modified on the engine. It is a stock 2005 Black Scorpion.

How do you check for exhaust leaks? What do you mean by "if there is no back pressure then it can cause misfire also?" Does this mean that an exhaust leak removes back pressure and can cause a problem?

Rob you didn't answer my question regarding clarifying the issue. Backfire or misfire? I guess I misunderstood, I thought you said a leakdown test, but you actualy said dropdown. Pulling one wire at a time is not really going to tell you much. You need to start with a leak down test of each cylinder. This is done with a leakdown tester and will accurately diagnose the mechanical condition of the engine. But again even if it was a mechanical problem such as valves or gaskets/seals it most likely WOULD NOT be intermittant...

Sparky - On timing, I'm a little confused. The mechanic told me that timing is set at the factory and they don't mess with it. When this originally started, I thouught I had a timing issue. Everyone keeps telling me that the computer controls timing and that can't be the problem. How can a non-mechanic know if it is a timing isse?

I had another friend on the boat that is an auto mecahnic at the same time that the mercruiser mechanic was on the boat. We were at the lake under load. There is a small rumble out the back of the boat. I wouldn't call it a "pop", and it wasn't coming from the air cleaner. More out the exhaust. They said it was a misfire, not a backfire. This was based on the sound of the motor. The computer had some "misfire" mode that didn't detect anything. From what I understood, the mercruiser computer cannot isolate cylinders to detect where the problem is coming from.

Also I believe the module only controls the advance of the timing, not the initial timing. If the distributor has a bolt that you loosen and it turns then initial timing is adjustable. I would have to look at my 06 to see. You still need to run the leak down though. This is a crucial test when troubleshooting this type of issue.

Yeah, real easy. Clip the inductive lead of the timing light on #1 cylinder plug wire, and the red and black to battery and find the mark on the vibration dampner behind the lower crank pully. The dampner should have a line on it. You may need to put some white paint or chalk in the line to see it with the timing light. It's probably fine, but still need to check it. Again, I can't stress enought the need for the leak down test first so you're not chasing your tail with this problem.

I looked up leak down tests on the web. I did have a compression test done at one point and all cylindars were at 165 psi. I can see that a leak down test is more specific than a compression test. I assume that it would be smart to check initial timing before heading to the shop for a test like this.

The shop that did the tune up last season told me they adjusted timing. They said sometimes to get it right they need to go to the lake under load. I'm not sure they really messed with the timing, but I should check it. Is this an easy thing to do on the water?

The problem started last season with lots of loud rumbling. It just started. From good to bad. Took it in for a tune up and it came back with less volume to the noise, but it was still there. It needed a tune up (they said lots of corrosion in cap). Maybe they adjusted the timing incorrectly while it was in the shop? Is checking timing a simple task?

Well initial timing is checked with the engine warm and at idle. Can be done in the driveway with fake-a-lake. No need to be under load, or on the water. As I said I believe the advance curve timing for the distributor is controlled throught the ECM and not adjustable, unless the ECM can be re-flashed, or re-calibrated. There are timing lights that can also check the amount of distibutor advance at a given RPM, and this could also be done in the driveway, but these timing lights are a bit more expensive than basic ones.

If the compression came back good and the problem only occurs at certain specific times a CLT check is not going to do much since it is most likely ignition related (Intermittent valve train failures are very very rare). Injectors fail all the time too though. Ohm the injectors. Spec does not matter just that they are all the same.

From there any engine technician should know how to use a labscope. If not find a new technician that does. There is no way to work on modern engine systems without utilizing a scope to check components. A simple Kv test on the ignition system should tell whether there is a secondary ignition problem. A true scope does NOT miss anything. If it shows up in the secondary patterns, then they can current ramp the primary side to see if it is a coil breakdown. The outputs/inputs to/from the ECM can also be checked to determine if the problem is computer or sensor related. Current ramping the injectors would tell how they are really doing underload. There is also the simple injector buzz test for pressure drops, and a pressure hold test.

Inducing a little propane would richen the mixture up to see if it clears up, which will determine if it is a lean miss. Without hearing it I have no idea what is going on with the engine myself.

I would never recommend running any motor with a misfire. Imagine being kicked in the shin every time it misfires...it doesn't feel good and will do damage over time.

99% of the time reflashing a ECM is a good thing. They are just like MS Windows patches, but for your engine.

I had the exact same problem with my Indmar motor. We did most of the tests you are talking about with no conclusions.

After months and months of trying different things, one of the Indmar techs came up with an idea...It turned out being two of the vacuum hoses were reversed. We swapped them to their correct positions and everything went back to normal. During one of the tuneups, a mechanic accidently switched two vacuum lines?? Good luck.

Do the leak down test. A compression test really doesen't tell you a whole lot. I can get more information from connecting a vacuum guage to an engine than a compression tester. You must eliminate what's not the cause. With your description it's possible to be a number of things. You can gain piece of mind and eliminate a huge part of the diagnostic "pie" by doing the leak down...